1. Field of the Invention
The present invention relates to a fixing device applied to an image forming apparatus, which fixes a toner image on a recording material passing through a nip between a fixing roller and a pressing roller, and the image forming apparatus.
2. Description of the Related Art
Conventional electrophotographic image forming apparatuses, such as copiers, laser printers, plotters, and facsimile machines, are configured to first read an original image by a scanner, and then expose the original image to light to form an electrostatic latent image on the electrostatic latent image carrier of the image forming unit. The electrostatic latent image is developed to a visible toner image. This toner image is transferred onto a recording material fed from the recording material supply unit. There are various types of recording materials such as recording sheets or envelopes. The transferred toner image is fixed onto the recording material to be discharged to the discharging unit or the like.
The image forming unit of such an image forming apparatus often uses a fixing device configured to apply heat and pressure by pressing the fixing member and the pressing member against each other. For example, Japanese Patent Application Laid-open No. H09-179435 discloses a conventional fixing device which includes a fixing roller as the fixing member with an internal heat source and a pressing roller as the pressing member. The fixing roller and the pressing roller are first brought into pressurized contact with each other and rotated. The rollers sandwich a recording material arrived at the pressurized contact portion or at the nip defined by the rollers and feed it at a predetermined speed. At this time, the rollers heat and press an unfixed toner image onto the recording material to fix the toner image as a visible image on the image surface.
Incidentally, it is effective to control the linear speed of each of the front and rear surfaces of a recording material to achieve a higher degree of flexibility related to fixing performance to fix a toner image reliably onto the image surface of the recording material depending on the type of the recording material and feed it at a predetermined speed in the discharge direction.
In the fixing process by application of heat and pressure, for example, such a fixing device is used in which either one of the fixing roller and the pressing roller is rotationally driven by a driver as a driving roller, and the other is driven by the driving roller.
In this case, when a thin sheet (recording material) having a basis weight of, for example, 65 g/m2 or less passes through the nip defined between the rollers, a slip occurs on the driven side. Besides, when a recording material made of two or more sheets combined together such as an envelope passes through the nip, the front and rear sides of the envelope facing the driving roller and driven roller, respectively, are fed at different speeds (linear velocities). This may cause stoppage of feed, unsettlement of images, feed wrinkles.
On the other hand, when a thick sheet having a basis weight of 200 g/m2 or more passes through the nip, neither slip on the driven side nor relative variations in feed speed (linear velocity) on the front and rear sides of the sheet occurs. However, the fixing member and the pressing member may define a curved nip, which causes curling of the sheet and may degrade the feed performance and stackability after printing.
To ensure that an image can be fixed on various types of recording materials (types of sheets) without slippage in fixing and rubbing of images, several attempts have been made. As examples of such attempts, the fixing member and the pressing member are driven via an idler wheel by the same driving source. The shape of the nip is flattened depending on the material and shape of the fixing member and the pressing member. The setting direction is regulated and the pressure is reduced depending on the recording material.
Japanese Patent Application Laid-open No. H9-179435 discloses an example of the technology. According to the conventional technology, when recording materials sequentially pass through the pressurized contact portion between the heating member and the pressure contact member, a drum effect increases by an increase in temperature on the axial periphery of the pressurized contact portion through which no recording material passes. At this time, the pressure applied by the pressure contact unit is reduced to thereby decrease the relative speed difference between the heating member and the pressure contact member. This alleviates the drum effect, thus preventing images from being rubbed or granulated.
Japanese Patent Application Laid-open No. 2002-351237 discloses another conventional technology in which a first fixing unit and a second fixing unit are arranged in the feed direction. The first and second fixing units are provided on the pressing roller side with a driving unit. The driving unit transmits a driving force in the form of frictional force to the heating member and the fixing roller, thereby causing them to be rotationally driven to sequentially fix an image and ensure the quality of gloss of the image surface.
Japanese Patent Application Laid-open No. 2007-310129 discloses conventional image forming apparatus including a fixing film that slides along a heater holder that integrally includes a heater and is driven by the rotational force of the pressing roller. In the image forming apparatus, a slip may occur in the flat nip between the pressing roller and the fixing film at a low temperature of the pressing roller in pressurized contact with the fixing film, while a slip occurs less at a high temperature. Further, a sheet is not pulled when a loop is detected which is caused by a slack of the sheet between the fixing nip and the transfer nip located upstream thereof on the feed path, while when no loop exists and the sheet is pulled, a slip is likely to occur during fixing. Taking these characteristics into account, slip prevention is performed when a slip is likely to occur during fixing and otherwise unnecessary slip prevention is not performed.
With the above conventional technologies, a driving force of the pressure contact member (pressing member) is transmitted to the heating member (fixing member) via an idler wheel to drive the fixing member and the pressing member together with the driving force from the same driving source. However, only this configuration is not enough to offer constant flexibility during fixing of images on various types of recording materials (types of sheet) to, for example, prevent slip during fixing and rubbing of images as well as to maintain durability without reducing the service life of the fixing member and the pressing member.
In the case of the image forming apparatus as disclosed in Japanese Patent Application Laid-open No. 2007-310129 having a flat nip and the fixing film to be rotationally driven, if a low heat capacity member is used for the fixing unit to achieve the recent trend of energy saving, it is difficult to form the nip with a width enough to ensure a sufficient amount of heat. Moreover, a drop in pressure requires to drop the linear speed accordingly to ensure a necessary amount of heat, thereby hampering productivity.
Furthermore, it is inconvenient that there are restrictions on the setting direction of the recording material and a particular type of sheet such as an envelope, the setting direction of which cannot be selected, cannot be handled.